The sales of articles such as vitroceramic cooking plates (or hobs) have been increasing constantly for several years. This success is explained, in particular, by the attractive appearance of these plates and the ease with which they can be cleaned.
It may be recalled that a vitroceramic is based on glass, referred to as precursor glass (or mother glass or green glass), the specific chemical composition of which makes it possible to induce controlled crystallization by suitable heat treatments, referred to as ceramization. This partly crystallized specific structure imparts unique properties to the vitroceramic.
There are currently various types of vitroceramic plates, each variant being the result of significant studies and numerous tests, given that it is very difficult to make modifications to these plates and/or the method by which they are obtained without risking an unfavorable effect on the desired properties: in order to be used as a cooking plate, a vitroceramic plate must generally exhibit transmission in the visible wavelength range which is both low enough to mask at least some of the underlying heating elements when off and high enough so that, depending on the case (radiant heating, induction heating, etc.), the user can visually detect the heating elements which are on with a view to safety; it must also exhibit high transmission in the infrared wavelength range, particularly in the case of plates with radiant heat sources.
Most contemporary plates are dark in color, in particular black, colored for example by using vanadium oxide added to the starting materials of the mother glass before melting; after ceramization, this oxide imparts a strong orange brown tint associated with reduction of the vanadium. Other colorants may also be used, such as the oxides of cobalt and manganese. With a low transmission coefficient below 600 nm, these plates make it possible above all to see red elements as heating elements heated to high temperature, or alternatively luminous displays based on red-colored monochromatic light-emitting diodes. There are also more transparent vitroceramic plates (such as the vitroceramics KeraVision or KeraResin, marketed by the company EuroKera), or other plates (such as the Keraspectrum vitroceramics), making it possible to display other colors.
The luminous illumination used with vitroceramic plates can make it possible to indicate controls or heating zones, to display data, to show decorations, etc. The light sources used are generally installed below the plate, in such a way that they can illuminate patterns (for example an icon or a symbol indicating that the heating zones are on, or an ON/OFF pattern), generally deposited by screen printing on the upper face of the vitroceramic. In general, it is desirable for these patterns, which are generally small, to appear with sufficient clarity and contrast and for them to be easily locatable, whether they are in the activated state (lit) or the deactivated state (off).
It is known to deposit patterns, intended to be illuminated, by screen printing with the same type of enamel as that conventionally used for the cooking plate decorations on the upper face of the vitroceramic, for example an enamel comprising 70% by weight of glass frit and 30% by weight of black pigments, the glass frit being in particular based on silicon dioxide SiO2 (present for example at 42% by weight of the frit), boron trioxide B2O3 (for example at 28% by weight), and aluminum oxide Al2O3 (for example at 18% by weight). However, when the light source arranged on the lower face of the vitroceramic is lit (emitting for example red light), the contours delimited by the screen-printed enamel may not appear sharp because the light diffuses through the enamel, thus degrading the definition of the pattern represented. The points on the lower face of the vitroceramic may furthermore be visible in the patterns by transparency, and may induce optical distortion of the light of the light source.
One way of overcoming these problems consists, in particular, in depositing black paint by screen printing on the lower face of the plate, facing the pattern of the upper face. It is also possible for a resin with the same refractive index as the vitroceramic to be combined with the black paint in order to limit the effect of the optical distortion due to the points. However, alignment of the enamel-based pattern screen-printed onto the upper face of the vitroceramic with the paint-based pattern screen-printed onto the lower face of the vitroceramic, this alignment being necessary in order to obtain the desired results, is very difficult to carry out industrially.